1. Field of the Invention
This invention relates to wastewater treatment systems and in particular to an apparatus and method for denitrification of nitrified wastewater or nitrified water with detergent or detergent-like compounds comprising available carbon.
2. Description of Related Art
Excessive concentrations of nitrogen in the form of nitrite and nitrate in water in the environment or in water discharged to the environment has both health risks to human beings and potential adverse consequences to aquatic environments. Nitrate is in the form of NO3 and Nitrite is in the form of NO2. Water with nitrate or nitrite concentrations is sometimes called nitrified water. Both are oxidized compounds of nitrogen. These compounds are considered inorganic forms of nitrogen.
According to Federal Environmental Protection Agency (EPA) Drinking Water standards, the Maximum Contaminant Level is 10 milligrams per liter of nitrate nitrogen or NO3 for safe drinking water. For estuarine environments, concentrations above 0.32 milligrams per liter of nitrate nitrogen or NO3 can cause adverse impacts from excessive algae growth. Concentrations above 0.5 milligrams per liter of NO3 have been found to accelerate eutrophication of coastal waters. Significant efforts are being made to remove and reduce the concentrations of nitrates and nitrites in water.
A denitrification process can be used for any nitrified water. Typical nitrified water is treated wastewater, ground water, process waters and some runoffs. Denitrification in nature occurs when water with concentrations of nitrite and nitrate are introduced into an anaerobic environment with a carbon source available. The carbon material is an electron donor. Denitrification in man-made processes traditionally involve a variety of carbon sources.
The prior art involving denitrification of treated wastewater use methanols and similar organic compounds. Such compounds are carcinogenic and explosive. These compounds require special handling, special construction and pose a threat to the environment from accidental release. Denitrification systems using methanol and similar compounds require venting to release an excessive mass of methanol. The use of methanol and similar compounds in treating wastewater may require the addition of a base substance to bring the effluent pH to near neutral values.
Various types of carbon sources have been investigated by many parties. U.S. Pat. No. 5,342,522, issued Aug. 30, 1994 to Eric H. Maraman et al., discloses a method for the treatment of sewage effluent comprising a step of denitrification using a carbon source such as the addition of methanol or internally generated fatty acids, for example, acidified surplus sludge. The carbon source must be well matched to the nitrate content of the effluent in order to prevent nitrate passing into the effluent. However, the use of methanol involves handling and storage of a poisonous, hazardous and explosive liquid. The use of internally generated fatty acids involves a physical effort to obtain the carbon source from the process and is complicated requiring a very skilled practitioner. The denitrification system in the Maraman patent requires that the fluid in the denitrification vessel be stirred by a paddle. Such paddles involve the use of energy and involve inevitable maintenance effort in a hazardous environment.
U.S. Pat. No. 5,588,777 issued Dec. 31, 1996 to Rein Laak discloses a wastewater treatment system that uses soaps for denitrification purposes in two different designs of wastewater systems. However, soaps are a processed animal fat, and certain soaps comprise significant concentrations of total nitrogen. This patent describes a dosing of soap as a carbon source in a rock layer in the bottom of a stratified filter. The Applicant, Michael B. McGrath, built a similar physical environment for denitrification in 1985 whereby, in constructing a Residential RUCK TM system, a mixing tank in which soap (from separated plumbing) was added to nitrified water which fluid then drained to a chamber filled with rock. Denitrification occurred in the mixing tank and in the rock filled chamber. Within a year, the voids in the rock chamber completely filled with a bacterial growth and completely clogged. The Laak patent relies on a similar situation where denitrification would occur in voids in a stone layer. The Laak design is such that if clogging does occur, there is no physical access to observe if that clogging is taking place and no access to clean the clogging should it occur.
In another system referred to as the Lunenburg RUCK CFT system, dishwashing liquid soap was used as a carbon source and it was unsuccessful as a carbon source. The denitrification portion of the system did not effectively denitrify. The Total Nitrogen concentration of that soap exceeded 129 milligrams per liter. The soap added nitrogen to a system designed to reduce nitrogen. A test was made of various laundry detergents by adding the manufacturers suggested amount of detergent to a standard wash in a washing machine and the measured concentrations of Total Nitrogen were in the range of 1 to 2 parts per million or milligrams per liter.
U.S. Pat. No. 5,185,080, issued Feb. 9, 1993 to Gregory Boyle discloses a system to denitrify wastewater by introducing bacteria and a source of carbon such as from milk or milk by-products into a detention treatment chamber and maintaining the temperature above about 48–50 degrees Fahrenheit. Boyle describes a pelletized form of carbon introduced daily. In the Boyle patent, the nitrate concentration is described as being more or less constant, the temperature of household effluent is described as always at about 48–50 degrees, denitrification is complete in four hours and the government must be a co-permittee in the operation and maintenance of an on site denitrification system. The carbon form in a pelletized form allows no flexibility in the mass of nitrogen removed during denitrification. The mass of nitrogen cannot be adjusted. The fixed mass of the pelletized detergent provides no ability for the denitrification portion of the treatment system to respond to variations in the daily flow of the treatment system.
Nitrate concentrations in residential wastewater are not constant. The concentration of nitrogen in wastewater has increased due to changes in the materials in household chemicals and the change in volume caused by low flow plumbing fixtures. Various parties promoting alternative innovative on site systems in the Commonwealth of Massachusetts have measured total nitrogen in household wastewater in reports to the Massachusetts Department of Environmental Protection. In several reports, the average influent total nitrogen concentration in household RUCK systems exceeded approximately 70 parts per million. The description of the Boyle system does not provide a system capable of treating higher concentrations of nitrate in the nitrified water.
Further, the Boyle patent ('080) states that residential wastewater is always in a certain temperature range, namely 48 to 50 degrees Fahrenheit. Normally, that is true. However, there can be places where local conditions could cause the effluent to be lower in temperature than what is stated in the Boyle patent. In Lunenburg, Mass., at the Woodlands Condominiums, a series of sewer pipes and pump stations move wastewater to a central treatment facility called a Commercial Ruck system. The temperature of the effluent entering the treatment system can reach 48 degrees Fahrenheit. An aeration stage drops the temperature to 44 degrees Fahrenheit. At that temperature, denitrification will not take place. At that temperature, denitrification will not start when the system is initially started. These temperatures are lower than the temperature described in the Boyle patent.
The Boyle patent further discloses that denitrification takes only four hours and relates to the system described in that patent and to the quantity of the concentration of nitrate in the effluent. In actual field performance, the time for complete denitrification can take up to forty hours with nitrified water at 50 degrees Fahrenheit and with detergent as a carbon. The short detention time in the Boyle patent for denitrification will result in incomplete denitrification.
U.S. Pat. No. 4,039,438, issued Aug. 2, 1977 to Donald R. Anderson discloses a process for the anaerobic biological denitrification of water comprising contacting water containing nitrate and organic material with cellulosic substrate sufficient to support biological denitrification and maintaining the water at a temperature from 50 degrees Fahrenheit to about 120 degrees Fahrenheit for a period of time from a few minutes to about 24 hours to effect the denitrification. However, the process in Anderson's patent relies on the upwelling of nitrified water through a quantity of cellulosic substrate. The carbon source in the cellulosic substrate will be exhausted and is difficult to replace in traditional mixing chambers and detention tanks. Typical manholes used in traditional wastewater treatment structures are 24 to 30 inches in diameter. Pump chambers have rectangular openings called hatches, which typically are 48 by 48 inches. Such hatches can be installed in detention and mixing tanks. These openings are too small for the replenishment of the cellulosic material in relation to the volume of the nitrified water to be treated. Since the description of denitrification is by upwelling, a special tank or container must be constructed. Such special tanks will be expensive.